It's the same reason a group of people going to a restaurant is a "party of two" (or four or however many). Generally speaking, any individuals considering themselves a combined group can be referred to as a "party".

Because it's fun fun fun all the time.

It's "party" as in "group" or "You're the warrior, you're the cleric, you're the healer, and I'm the handsome and stealthy archer. This is going to be the best party ever." I imagine they chose the word "party" over "group" for the alliteration in "political party."

c.1290, "part, portion, side," from O.Fr. partie "a part, a party" (12c.), lit. "that which is divided," from fem. pp. of partir "to divide" (see part (v.)). Political sense of "side in a contest or dispute" evolved by 1300; meaning "a person" is from 1460. Sense of "gathering for social pleasure" is first found 1716, from general sense of persons gathered together (originally for some specific purpose, e.g. dinner party, hunting party). The verb is first attested 1922, from the noun. Phrase the party is over is from 1937; party line is first recorded 1834 in the sense of "policy adopted by a political party," 1893 in the sense of "telephone line shared by two or more subscribers." Party pooper is from 1951, Amer.Eng. http://www.etymonline.com/index.php?term=party The Party in political contexts usually refers to a dominant political party in a country, often the only party (in a single party systems). Commonly such term is used to refer to communist party in communist states. For example, in reference to the Soviet Union, the Party means the Communist Party of the Soviet Union; in reference to People's Republic of Poland it refers to the Polish United Workers' Party. http://en.wikipedia.org/wiki/The_Party_%28politics%29